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Defining Risk From
Meatborne Parasites

Examining an electrophoresis
gel, zoologist Benjamin Rosenthal
identifies fragments of genes
isolated from related species
of parasites. Genes common
to all such species provide
a good basis for comparing
and diagnosing them. (K9857-1)

A single-celled parasite called
Toxoplasma gondii likely infects more than 60 million people in the
United States, according to the Centers for Disease Control and Prevention.
While most of the infected don't have any symptoms, the parasite can cause
serious damage in developing fetuses, people with AIDS, and others with
compromised immune systems.

A common route of infection is exposure to egglike oocysts in the feces of
infected cats. But T. gondii can infect tissues of domestic and wild
animals, and people can ingest the parasite by eating or handling raw or
undercooked meats, including pork, lamb, or venison. According to rough
estimates, about 3 percent of U.S. market-age pigs have T. gondii tissue
cysts. But improvements in how swine are fed and housed are reducing exposure.

Zoologist Benjamin Rosenthal
inspects the pattern of colored
DNA fragments defining several
parasite genes while technician
Mayee Wong prepares to load
other samples into the sequencer.(K9856-1)

Still, "it looks like
foodborne infection is a major route," says Benjamin M. Rosenthal with the
Agricultural Research Service in
Beltsville, Maryland. He notes that members of a vegetarian religious
denomination living in suburban Maryland were one-half to one-tenth as likely
to have been exposed to T. gondii as their neighbors. And a large
European study attributed between one- and two-thirds of toxoplasmosis-related
birth defects to consumption of undercooked or inadequately cured pork, lamb,
or beef. Thorough cookingto 160-170°Fcan prevent transmission.

Toxoplasma parasites have close relatives that can also form cysts in
the muscles of domesticated animals and primates. These include parasites
belonging to the genera Neospora, Hammondia, Besnoitia, and
Sarcocystis.

"We know far less about their acotual or potential public health
risk,"says Rosenthal. "These other cyst-forming parasites may
contribute to the estimated 86 percent of U.S. foodborne illnesses that
currently go undiagnosed." There are many reports of Sarcocystis
infection in humans worldwide, he adds, but the animal sources of those
infections are poorly defined.

Microscopic cysts of Sarcocystis
hominis nestled in a
sliver of beef tongue. Proper
cooking prevents the parasites
bundled in these cysts (stained
dark purple) from infecting
people. Magnified about 400x.(K9871-1)

That's why Rosenthal, a molecular
systematist, joined ARS' Parasite Biology, Epidemiology, and Systematics
Laboratory nearly 3 years ago. He is defining the distribution of these
cyst-forming parasites in domestic and wild animals and in people. And he's
doing it by looking for variations in their genes.

"You have to know who's who before you can make effective risk
assessments," he says.

Until recently, for example, researchers didn't have sensitive tests to
diagnose or distinguish Neospora caninum infections from those caused by
T. gondii.

"Many of these other parasites also cause miscarriages in cattle, goats,
sheep, and nonhuman primates," Rosenthal says. "Do they pose a direct
risk to human health? To begin to answer the question, we first have to tell
them apart and determine how many species exist. Learning how they are related
to one another will help us assess their potential to infect people and cause
disease."

Genetic comparisons may also shed light on the evolutionary
age of these parasites and the conditions that let them flourish. For instance,
there are only three main genotypes of T. gondii worldwide. And there's
very little genetic variation within each genotype, which is unusual for such
an especially abundant organism.

These facts suggest that T. gondii may have relatively recent
evolutionary origins, says Rosenthal, and may have taken advantage of
opportunities for transmission that came with the domestication of animals.

"Parasite adaptations to particular agricultural practices may explain
why infection with T. gondii is so common in our cats and in certain
food animals," he says.

But such a conclusion will rest on a better understanding of the genetic
variation of other, related parasites, says Rosenthal. A recent study led by
Stanford University scientists may help explain T. gondii's success. The
team reported strong evidence that the most virulent of the three genotypes
appears to be the offspring of a sexual recombination between the other two.

In this case, it looks like Mother Nature selected a whole new strain rather
than selecting by individual gene mutations, says Rosenthal.

With Toxoplasma, he says, "every gene you look at exists in only
one or two forms. Is the genetic uniformity of T. gondii extreme when
compared to its closest parasitic relatives?"

Rosenthal's research will help answer that question while developing the
diagnostic tools necessary to evaluate any food-safety risks posed by these
"poorly understood but intriguing parasites and their
relatives."By Judy McBride,
formerly with ARS.